Demerara Sugar is defined as:
A white cane sugar treated with molasses to produce large, pale brown crystals.
It is often served with coffee and is used for making biscuits (cookies), cakes and confectionery.
Brown sugar or raw sugar can be substituted.
It was first produced in the Demerara region of Guyana, West Indies hence the name Demerara.
Also this website has more information on sugar than most I have seen.
http://www.sugarindia.com/Now for my own two cents.
I would note though that depending on the recipe you are using it might be better to use one over the other. For example US light brown sugar will tend to pack easier whereas sugar in the raw or pure cane will be more granular in texture.